Recommend some mosfets please.

Discussion in 'General Electronics Chat' started by jdraughn, Dec 21, 2010.

  1. jdraughn

    Thread Starter Active Member

    Jan 30, 2009
    33
    0
    I am looking to build multiple projects that require mosfets. CNC machine using bipolar stepper motors, quad rotor helicoptor, radio controlled lawn mower using a 24v motor from a electric scooter, ect...

    I am trying to find some decent, inexpensive mosfets. I was planning on building discreet totempole drivers. I still havint decided how I am going to go about driving the bipolar stepper motors, all the driver ic's for mosfet based hbridges seem pretty spendy.

    I am planning on ordering the fets from alliedelec.com(or is it alliedelectric.com)since I am ordering a few other parts from there anyway.

    It would be nice if they could turn fully on with 5v, but thats not a requirement. My stepper motors are rated for 36v, so 40v fets would be nice. I would prefer through hole but smd's would be ok if they arent too small. Bonus points if you could also recommend complementry pnp fets too.
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Just go to http://www.mouser.com and go through their drill down menu for their database, you'll find tons of acceptable parts you can use but you want those with high current and very low Rds specs.
     
  3. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    What Marshall said, plus don't pick just one part number and try to use it in every circuit you build. Match the specs of the Mosfet to the circuit.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If your steppers are rated for 36v, you will probably want to drive them using two to four times that voltage, along with a chopper driver. Otherwise, you'll find that your stepping rate and torque will be limited due to the inductance of the motor windings.

    You don't say much about the steppers. Usually, NEMA size 23 is about the smallest that people use for CNC machines. If you had selected lower voltage steppers (say, 3v to ~8v) you would have a much wider variety of modern MOSFETs to choose from.

    Besides N-channel or P-channel and logic-level or standard, there are three basic parameters MOSFETs have you need to be concerned about:
    1) Vdss - maximum voltage from the drain to source terminals.
    2) Rds(on) - Effective resistance from the source to the drain terminals when the gate to source voltage is as specified - lower is better.
    3) Qg - The total gate charge, specified in nC, or nanoCoulombs - lower is better.

    As the Vdss rating goes up, in order to keep the Rds(on) low, the gate charge increases, which means you need a more effective (higher current) gate driver.

    P-channel MOSFETs have largely fallen out of favor, as they require 2.5 times the channel area to be complementary to an N-channel MOSFET, so the gate charge goes up by that same amount. Modern designs use a high-side gate driver with N-channel MOSFETs.

    Without having more info on the motors (voltage, current), the supply voltages you'll be using and your space constraints, it'll be hard to suggest a "one size fits all" MOSFET.
     
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,006
    1,528

    I have used these drivers, that are not overly expensive; http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250589480445&ssPageName=STRK:MEWAX:IT

    The thing with making your own drivers for a CNC machine is that you have to come up with something that is compatible with the available CAD/CAM programs, or write your own that you can't use the commercial programs with. There are more and more hobbyist project that have the code already written to do the project that people are sharing.
    if your machine is proprietary you can't use that existing code.

    The drivers I linked to are capable of using Mach3 or EMC2. They use the standard, direction and motor speed signals. All you need to add is a transformer and break-out board to you computer. They are called "Dragon Drivers".
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,782
    941
Loading...